Method for manufacturing a coaxial filter



NOV. 3, 1959 CAPEN ETAL 2,911,333

METHOD FOR MANUFACTURING A COAXIAL FILTER Filed Nov. 24, 1954 INVENTORS H-NUEMAN CAPE/Y CARL R. BROWN BY LEE F. E VANS MAW 7 2,911,333 Patented Nov. 3,1959

FOR MANUFACTURING A COAXIAL FILTER Harold Norman Capen, Cedar Grove, Carl R. Brown, Fairlawn, and Lee F. Evans, Bloomfield, N.J., assignors to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Application November 24, 1954, Serial No. 470,954

2 Claims. (Cl. 1542.25)

METHOD This invention relates to filter devices and methods for making them and more particularly to distributed parameter coaxial filters particularly adapted to be manufactured by molding techniques.

Conventional distributed parameter low-pass T-section or pi-section filters utilize concentric metallic members as the capacitor elements and sections of the inner conductor of a coaxial type filter as the inductance elements for use at high frequencies. It is important that the filters be carefully designed so that the measured pass band response conforms to the desired calculated response very closely. At elevated frequencies, such as at super-high frequencies, mechanical tolerances become critical for maintaining desired electrical values. In the past, it has been found that in order to maintain the desired frequency response within close limits, precision machining involving working to narrow tolerances has been required. Such precise machining operations are extremely time consuming, and consequently the filters produced are quite expensive.

It is an object of this invention .to provide a coaxial filter of the distributed parameter type suitable for use at elevated frequencies.

It is another object to provide a method of manufacture for such filter elements that is simple and inexpensive compared with presently available methods.

It is a feature of this invention that a coaxial filter of the distributed parameter type is provided by molding two half-sections of a polymeric material in such a manner as to provide axially spaced cavities therein. The surfaces of these cavities are then metallized, and the corresponding metallized surfaces of the two halves are electrically connected together about an inner conductor. In this manner, the separate provision of precisely machined annular capacitive elements about a central conductor, as has been used heretofore for high-frequency filters, is eliminated.

For a better understanding of the invention together with other objects and features thereof, reference is had to the following description taken in conjunction with the accompanying drawings wherein:

' Fig. 1 is a view in elevation of the filter structure;

Fig. 2 is an axial sectional view of the filter device taken along the lines 2-2 of Fig. 1;

Fig. 3 is a longitudinal sectional view of a halfcylindrical section of the filter showing the axially disposed cavities and interconnecting axial grooves;

Fig. 4 is an enlarged cross-sectional view taken along the lines 4-4 of Fig. 2;

Fig. 5 is an enlarged cross-sectional view taken along the lines S5 of Fig. 2 showing a section through a cavity; and

Fig. 6 is an enlarged cross-sectional view corresponding to Fig. 5, but showing an embodiment of a solid capacitive unit within a cavity.

The filter of the subject invention behaves electrically in a manner similar to conventional coaxial-type lowpass filters of the distributed parameter type; its struc- I ture, however, ofiers considerable advantages in ease of tion to a waveguide or coaxial transmission line.

fabrication and in being readily adapted to manufacture by mass production techniques.

Referring to Fig. 1, the body of the coaxial filter structure-1 consists of two similar halves 2 and 3 of a dielectric material joined together. A pin contact 4 and spring contact 5 are provided at opposite ends of the filter structure 1 for interconnection with a filter holder. Other appropriate connectors may be used for connec- Close- I fitting sleeves 6 are also provided at opposite ends of the filter structure for making electrical connection to a coaxial filter holder. For certain applications these sleeves may be made coextensive with the filter structure thereby serving as an outer conductor.

Referring to Figs. 2 and 3, the filter structure 1 is preferably made by molding a half cylinder 2 of a high polymeric material in a die, the die structure shaping the material to provide axially spaced cavities 7 therein. The dielectric material used should be readily moldable by known molding techniques, such as compression molding, transfer molding or injection molding, and should have a low electrical loss factor at high fre-, quencies. While materials such as polystyrene, cross linked polystyrene, polyethylene and polytetrafluoroethylene are available for this purpose, a moldable low-loss dielectric'material such as polymonochlorotrifiuoroethylene is preferred. Such a material is commercially available as Du Pont Kel-F. The cavities 7 are usually regularly spaced axially and generally of cylindrical 1 practice of this invention.

cross section. These cavities may vary considerably in number depending upon the degree of filtering desired and other design considerations. In general the provi sion of from four to ten cavities is preferable in the The cavities located at the extreme ends of the cylinder are generally somewhat smaller dimensioned than the other cavities to compensate for end electric field effects. After a first half longitudinal cylindrical section 2 has been molded with the desired axially shaped cavities 7 provided therein, the walls of the cavities are metallized so as to provide hollow capacitive units 8 therein. An inner conductor 9 is placedaxially in electrical contact with portions of the metallized surfaces. The second half-cylindrical section 3 is then electrically connected to the first halfcylindrical section 2 by soldering or otherwise joining the corresponding metallized portions of the cylinders. In this manner hollow conductive units 8 are provided electrically connected to the inner conductor 9 and disposed about it. An outer conductor 10 is concentrically disposed about the outer surface of the cylinder 1. This outer conductor 10 may consist of a tight-fitting conductive sleeve, such as sleeve 6 extended to be co-extensive with the cylindrical filter structure, or, preferably, a deposited metallic layer. An adherent film of silver is considered desirable forthis coating.

In Fig. 3 axially disposed grooves are shown interconnecting the cavities 7. Upon metallizing the wall portions of these cavities 7 and the grooves 11 with a conductive material, conductive elements co-acting to form a distributed-parameter filter element are thereby provided. By providing these grooves 11, a separate inner conductor is no longer required.

In Fig. 4 is shown an enlarged cross-sectional view taken along the lines 44 of Fig. 2. The outer conductor 10 and inner conductor 9 are shown therein in coaxial relationship to the half-cylindrical section 2 of dielectric material.

In Fig. 5 is shown an enlarged cross-sectional view taken along the lines 55 of Fig. 2. This section is taken through one of the cavities 7. In the embodiment shown, the conductive elements are shown as hollow conductive units 8 conformably disposed within the cavities 7, the wall portions of the conductive units conforming to. the wall portions 12 of the cavities 7. Upon placing an elongated inner. conductor 9 in contact with the surface of these hollow units 8, electrical contact is thereby readily made.

In Fig. 6 is shown a partial sectional view of another embodiment of this invention. As shown in this figure, the conductive elements are solid conductive units completely filling. the cavities 7.

It will be understood that many methods well known to the art are available for metallizing non-conductors. While all are not equally suitable where different polymeric materials are used, 'the preferred methods for given dielectric materials may be readily determined.

Thus, in one method the internal cavities 7 and the outer surface of the dielectric material may be rendered conductive by use of graphite. This may be done by dry brushing the graphite or by use of a suspension of graphite in water. After the surface has been rendered conductive, it may be copper-plated followed by silver plating or it may be silver-plated directly. In general, for the conductive elements and the outer conductor the use of a metal such as silver is preferred. Other metals such as platinum, rhodium, tin, and various alloys that are corrosion resistant and of high conductivity may also be employed. A silver film may also be deposited by chemical reduction of a silver nitrate solution, as is well known in this field. Various other techniques, such as the application of metallic paints, cathodic sputtering and metal spraying are also considered useful for this purpose. Where plating methods are used, a preliminary silver strike is frequently employed.

Where the cavities are completely filled by solid conductive elements, this may equally well be accomplished by any of the known techniques used for metallizing non-conductors. It is also considered feasible to incorporate the solid units in the filter structure as inserts during the molding process.

While it is understood that the electrical properties of the filter structure are a function of the specific dimensions employed, which determine the frequency and band-pass characteristics, by way of illustration the following dimensions are shown for a low-pass filter useful in the super-high frequency range at a frequency of approximately 10,000 megacycles per second. It will be understood that the coaxial filter structure shown is of the distributed parameter type, i.e., it contains distributed inductance and capacitance along its length. The relatively thin elongated wire sections are chiefly inductive while the conductive elements, hollow or solid units, conformably disposed within the cavities are chiefly capacitive. For a filter structure having six sections, an over-all length of approximately 2 inches is used. The inner conductor is approximately 0.032 inch in diameter with an average cavity having a cross-sectional diameter of 0.095 inch. The Width of such a cavity is approximately 0.065 inch.

While we have given dimensions for our filter structure for use at super-high frequencies, it is readily apparent that the method described herein may equally well be employed to provide filter structures usable at other frequency ranges. Also, while we have described above the principles of our invention in connection with specific apparatus and methods, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our inventionv as set forth in the objects thereof and in the accompany ing claims.

We claim: v 1. A method for manufacturing a coaxial filter com-.

prising molding a first half-longitudinal cylindrical section of a moldable polymeric material, said material being shaped to provide axially spaced cavities therein, metallizing the internal surfaces of said cavities, placing an elongated electrical conductor axially in electrical contact with portions of said metallized surfaces, providing a second half-cylindrical section having corre-. sponding metallized cavity surfaces to mate with said first section and electrically connecting said metallized portions of said. half-cylindrical sections to thereby provide unitary conductive elements electrically connected with said inner conductor, and disposing an outer conductorconcentrically about the outer surface of said cylinder.

2. A method as in claim 1 wherein said moldable polymeric material comprises polymonochlorotrifluoroethylene.

References Cited in the file of this patent UNITED STATES PATENTS 2,438,913 Hansen Apr. 6, 1948 2,521,843 Foster Sept. 12, 1950 2,553,312 Gurewitsch May 15, 1951 2,574,790 King Nov. 10, 1951 2,641,646 Thomas June 9, 1953 2,700,136 Devot Jan. 18, 1955 

